Several commercial devices of the sonar kind are used in the exploration of the seabed and of its underlying layers, especially in the field of marine geology, in which a large number of researches attempt to obtain information concerning the nature and distribution of sediments. It is well known that the behavior of acoustic waves propagating through sediments is more complicated to understand, and, specifically, the absorption of acoustic energy is higher than in the water column, especially in the frequency band corresponding to higher frequencies. Traditional acoustic systems seem inadequate to fulfill all possible environmental situations that may be encountered in the exploration of the upper layers of marine sediments, and this is true in particular during prospecting for archaeological research purposes, for which nowadays no high-resolution acoustic devices exist that have an adequate capacity of penetration into the sediments. To be able to carry out this kind of research, two important prerequisites exist with regard to the acoustic prospecting pulse, although, unfortunately, they are often antithetic: 1) a high frequency is required in order to obtain a high resolution, so as to be able to visualize irregularities that may even have small dimensions and that are hidden in the upper layers of the sediment; and 2) an adequate penetration efficiency is required to perform the detection at greater depths, and this means that lower frequencies and a higher acoustic power must be used. Therefore, one should have at his disposal an acoustic source with a large-band frequency spectrum (e.g. from some kHz, up to some hundreds of kHz), and at the same time, a suitable acoustic power. Actually, commercially available acoustic systems do not fulfill both of these requirements at the same time, and consequently they cannot be utilized in an ample range of experimental conditions and marine environmental situations.
For what concerns a parabolic acoustic source of the “sparker” kind—which forms part of the above-cited echographic system, and which, taken alone, has already been disclosed in other patents of the prior art (see Cannelli G. B., D'Ottavi E. and Santoboni S., Electroacoustic pulse source for high-resolution seismic prospecting; U.S. Pat. No. 4,734,894; Canadian Patent 1 250 040; EP 0 230 415; JP Patent 8755/92)—while being adequate to focalize the acoustic wave generated by the electric spark, so that the signal transmitted along the principal axis is characterized by a high power in a certain frequency band, it seems unsuited for marine prospecting, because of the following reasons. The acoustic wave generated by that source is characterized by a “primary pulse” which is followed by one or more secondary pulses due to cavitation bubbles that randomly appear in the time domain. This makes it impossible to discriminate the pulses reflected by the seabed, ascribable to objects buried in the sediments, from the signals due to cavitation and which appear as noise. This drawback does not prevent the preceding echographic system from detecting antique cavities formed in the soil, since it suffices in any case—in order to detect the cavities—to receive a signal reflected by the air-soil interface of the cavity, although this signal is disturbed by the cavitation effect (see for instance FIG. 7 of U.S. Pat. No. 4,899,845, issued to the same inventors). The same inventor of the apparatus of the present application therefore tried—together with others—to minimize the cavitation signals with respect to the primary signal by including in the ecographic device, in place of a single parabolic source, a suitable tuned array of parabolic sources which had also been disclosed in several “national” patents [Cannelli G. B. and D'Ottavi E., Method of high-resolution sea bottom prospecting and tuned array of paraboloidal electroacoustic transducers to carry out such method; JP Patent 5-505235, U.S. Pat. No. 5,398,217, European 0 491 775; Canadian Patent 2,065,457]. By this array one obtains an improvement of the signal in the time domain, but it shows drawbacks such as a lower acoustic efficiency (defined as the ratio between the emitted acoustic energy and the electrostatic input energy), and a considerably limited frequency band that is insufficient for a high resolution, though the latter is indispensable to detect small findings hidden in the upper layer of the marine sediment. Moreover, this apparatus has an excessive cost because it requires a high number of transmission transducers.